Ah nice, this one has some depth to it. I'm assuming you want the horizon line to be where ground meets the sky here, and that those shapes in the back are distant mountains. If that's the case, than the far trees are way to large, something that close to the horizon (basically in line with the mountains) should be smaller than a pixel at that distance (how big would a tree be standing next to the mountains?). Secondly, If you want to increase the sense of depth: Add some light blue haze to the ground as it gets closer to the horizon. This represents all of the atmosphere between the viewer and the landscape as it falls into distance. Just a few points to help you along, but I really like the clarity of this piece otherwise!

Thank you both for your comments. This is a naif picture so i decided to joke just a little with proportions ;-) anyway thank you @Papercut for suggesting me something useful.